1. Technical Field
The invention relates generally to charged particle beam technology, and more specifically, to charged particle beam projection systems.
2. Background Art
The proliferation of modem electronics can be traced back in time to the invention of the integrated circuit, or semiconductor "chip". Over the years, advances in integrated circuit manufacturing processes have greatly increased the functionality of chips. A typical chip is fabricated on a semiconductor substrate, and patterns of conducting layers and insulating layers are formed by a series of deposit and etch process steps.
The generation of the pattern on the semiconductor substrate on any of these layers is referred to in the industry as "lithography". Lithography is the exposure of a resist (a radiation-sensitive thin film covering the substrate) with an energy-bearing radiation. The radiation causes a change in the chemical properties of the resist so that the areas in the film, which were exposed to the radiation, respond to the application of a developer substance differently than those areas that were not exposed. Depending on the resist material, this response could be either a reduced or increased solubility in the developer ("negative" or "positive" resist, respectively). Nonetheless, both cases create a relief or topographical structure in the resist layer, such that radiation exposed areas of the substrate are resist free after development or visa versa. The remaining resist then acts as a mask for subsequent deposition or etch processing.
High volume production of ultra large scale integrated (ULSI) circuitry typically uses visible and ultraviolet light (UV) in a projection/light-optical system. In projection systems the reticle contains the pattern of the entire chip or large portions of it, large compared to the critical dimensions (CD) by at least three orders of magnitude. However, this technology eventually reaches a fundamental limit of application to lithography of patterns with ever smaller dimensions. Techniques envisioned to replace the use of UV light include x-rays as well as charged particle beams (i.e., ions and electrons).
Charged particle beam lithography has been in use almost exclusively with using probe beams, i.e., beams with a diameter not exceeding a few micrometers. The technique of a probe beam system is comparable to drawing a pattern with a pen. Although most of the tools of the probe beam system do project the image of a stencil reticle reduced in size onto the substrate, this reticle does not contain a "pattern", but only a single (rectangular) area. This single area imaging represents a fundamental difference between the probe beam system and projection systems.
Charged particle projection systems are considered an alternative for the follow-on technology of light optics. While in principle, charged particle systems operate in a manner quite similar to a light-optical system, charged particle systems require special techniques for calibration and alignment. Some techniques for calibrating and aligning a specific probe-forming charged particle beam system, e.g.,the electron beam or "E-beam" system, are found in the following U.S. Patents: U.S. Pat. No. 4,675,528, "Method for Measurement of Spotsize and Edgewidth in Electron Beam Lithography", issued June 1987 to Langner et al.; U.S. Pat. No. 4,503,334, "Method of Using an Electron Beam", issued March 1985 to King et al.; U.S. Pat. No. 4,737,646, "Method of using an Electron Beam", issued April 1988 to King et al.; U.S. Pat. No. 5,466,549, "Method of Detecting and Adjusting Exposure Conditions of Charged Particle Exposure System", issued November 1995 to Yamada; and U.S. Pat. No. 5,438,207, "Electron Beam Direct Writing System for ULSI Lithography with Facilitated Rotation and Gain Corrections of Shot Patterns and Electron Beam Direct Writing Method for Same", issued August 1995 to Itoh et al.
One specific method to focus patterns on the reference target and/or to correct system problems, such as rotational errors, is disclosed in the aforementioned Langner et al. patent. The system discussed in the Langner et al. patent scans a beam spot across the sharp edge of the reference target to measure the image quality of the spot generated by the probe-forming system in terms of size, shape orientation, and edge acuity. The beam spots generated by the probe-forming system must meet certain specifications within very close tolerances. The process of scanning a beam across a sharp edge of the reference target and measuring the beam current, the current measured from the electrons, found on the surface of the reference target is known as knife edge scanning (KES).
Although the above-mentioned KES techniques are used successfully in probe-forming systems, there is not enough beam current density for precision measurement of image quality in a large-area projection system. That is, despite the higher total beam current in a projection system (more than an order of magnitude), the current in a section of the image on the reference target commensurate with the CD in the pattern is low. Hence, the signal-to-noise (S/N) ratio is so small that all the aforementioned methods are rendered ineffective in a projection system.
Accordingly, a need has developed in the art for a method and system for measuring system performance with respect to image quality in large-area charged particle projection systems.